Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $246.76   13.686%   $280.5316   $280.53   $280.55   $280.50  
  Purchase #2 $210.66   11.816%   $235.5516   $235.55   $235.55   $235.60  
  Purchase #3 $150.72   13.630%   $171.2631   $171.26   $171.25   $171.30  
  Purchase #4 $104.64   12.302%   $117.5128   $117.51   $117.50   $117.50  
  Purchase #5 $159.65   10.260%   $176.0301   $176.03   $176.05   $176.00  
  Purchase #6 $48.57   6.656%   $51.8028   $51.80   $51.80   $51.80  
  Purchase #7 $92.70   8.766%   $100.8261   $100.83   $100.85   $100.80  
  7 purchase totals = $1,013.70       $1,133.5181   $1,133.51   $1,133.55   $1,133.50